* [R] Add a compatibility layer to load Booster from an old RDS
* Modify QuantileHistMaker::LoadConfig() to be backward compatible with 1.1.x
* Add a big warning about compatibility in QuantileHistMaker::LoadConfig()
* Add testing suite
* Discourage use of saveRDS() in CRAN doc
* [R-package] replace uses of T and F with TRUE and FALSE
* enable linting
* Remove skip
Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>